Various aluminum containing compounds are used as precipitating agents in sewage treatment plants. One of the most widely used chemicals for the treatment of water is aluminum sulplate, widely known in the trade as Alum. These compounds are specifically used as flocculating and coagulating agents in water purification treatment. Although Alum has been extensively used, it presents several drawbacks namely its poor performances at low temperature, its high alkalinity requirements and potentially high residual soluble aluminum compounds.
The recent development of basic poly aluminum sulphate has provided products which overcome most of the difficulties mentioned for Alum. However, a major problem associated with the use of basic aluminum sulphate is the stability of the solutions. The difficulty is that aqueous solutions of basic aluminum sulphate tend to form a precipitate of metal salt or become cloudy partly gelatinous after only a short period of time. When this occurs, these solutions can often no longer be used or are less effective in most applications. Therefore, unless they are stabilized in some manner, basic aluminum sulphate solutions must be used within a very short time of their preparation. This is dearly a serious disadvantage because most industries require chemicals which are stable over a long period of time so that they can be stored in reasonable quantities and used as and when desired.
The traditional methods of preparations of polyaluminum sulphate solutions usually follow a partial neutralization of aluminum sulphate with hydroxyl groups from lime, caustic soda, soda ash, ammonium hydroxide or other alkali sources to a pH of approximately 3.5-4.3, typically 3.8, since aluminum hydroxide is not precipitated below a 3.8 pH.
Stablizers such as phosphates or chlorides may also be added to partially replace sulphate groups, or alternatively organic complexing agents such as sodium heptonate, citric acid, sorbitol, sodium citrate, sodium tartrate, sodium gluconate and the like may be added separately to stabilize the aqueous aluminum salt. The stabilization and neutralization techniques are examplified in Canadian Patents Nos. 1,123,306, 1,203,364, 1,203,664 and 1,203,665, as well as in U.S. Pat. Nos. 4,284,611 and 4,536,384.
One will usually encounter an important by-product loss when using the processes described in the prior art. Compounds such as calcium or sodium sulphate and ammonium sulphate in concentrations that will range from 20 to 30% by weight will typically be produces as by-products. The exact percentage of loss will depend on the basicity of the solution produced and on the source of alkali used. Also, mixing and possible filtration problems occur when lime is used as the alkali. Finally, possible crystallization problems may occur when sodium sulphate is formed as a by-product.
Finally, one of the most important problems encountered in the storage of an aluminum based product such as poly aluminum sulphate is the precipitation of substantial amounts of aluminum hydroxide within 2 to 30 days following the preparation of the desired product, whether it is stablized or not. Although the rate of hydrolysis leading to the precipitation of aluminum hydroxide will vary depending on the method and temperature of preparation as well as the choice of the stabilizer, it is in most cases a major problem.
Therefore, it would be highly desirable to provide an aluminum based product useful as a water treating agent and storable for long periods of time without encountering major losses in efficiency.